1. Field of the Invention
The invention relates to a beam forming antenna system which provides the capability of producing multiple beams from an array of radiating elements. More specifically, the invention relates to such a system using beam forming networks and simple junctions.
2. Description of Prior Art
In many surveillance radar applications it is desirable to obtain not only the azimuth bearing of a target, but also its elevation angle, which can be used in conjunction with the range to calculate the height of the target. This may be achieved by using separate antennas to obtain azimuth and elevation angles, by using a phased array antenna with a narrow beam which is rapidly scanned in both azimuth and elevation to cover the surveillance area, or by using a multiple beam antenna which produces several beams from the same aperture, such beams typically being stacked above each other or arranged in a suitable three-dimensional arrangement.
This invention is particularly related to a beam forming antenna system including beam forming circuitry coupled to linear, circular, planar or three-dimensional (typically conformal) arrays to supply signals to the antenna elements so that multiple beams are formed on transmit, or to receive signals from the corresponding multiple beams.
The most well known example of prior art is the orthogonal beam forming matrix commonly known as the "Butler Matrix". This produces, for the transmit case, an aperture illumination with uniform amplitude distribution and linear phase distribution corresponding to the selected beam direction. A "Butler Matrix" with N antenna elements (N is normally a power of 2) may have up to N input ports, each corresponding to a beam direction which is orthogonal to (in a mathematical sense) and thus isolated from (in an electrical sense) the other beams. For a transmit/receive antenna, if less than N ports and beam directions are required, the remaining ports may be terminated by matched loads to maintain the properties of the "Butler Matrix". A disadvantage of the "Butler Matrix" is that it produces uniform amplitude aperture illumination for each beam, thus giving a beam with high near-in sidelobes. To overcome this problem "modified Butler Matrixes" have been described which give tapered amplitude distributions, allowing the essential properties of the network to be used for low-sidelobe multiple beam antennas. A further disadvantage of the "Butler Matrix" (and "modified Butler Matrices") is that some of the paths within the matrix cross over, thus making waveguide, stripline and microstrip implementations difficult.
A further example of a prior art beam forming network is given in U.S. Pat. No. 3,868,695, issued Feb. 25, 1975, to E. H. Kadak, which invention uses delay lines, connected to the signal ports by power dividers, and by further power dividers to the antenna elements. The description states that, for an 8 element antenna, an additional 9 dB of insertion loss is introduced, because of the use of matched, isolated power dividers before and after the delay lines (this additional insertion loss is 3 dB for each level of binary splitting in the power dividers). Because of the flexibility introduced by the use of the set of delay lines (typically coaxial cables), this beam forming network is appropriate for use on linear, planar or "conformal" arrays, with uniformly or arbitrarily spaced elements, whereas the "Butler Matrix" is suitable for linear or planar arrays with uniformly spaced elements.
Another example of prior art is the "Rotman Lens" (see, for example, Hansen, R. C. (ed) Microwave Scanning Antennas (Academic Press 1964), Vol. 1, Apertures, pp. 245-246, or, Rotman W. and Turner R. F. Wide-Angle Microwave Lens for Line-Source Applications IRE Trans., AP-11, November 1963, pp. 623-632), which may be used with linear or curved arrays to produce multiple beams in one plane. This has a planar lens structure, which is designed so that it only propagates TEM waves with linear dispersion characteristics. Waves are launched from one side of the lens, from positions corresponding to the required beam directions. Ports on the other side are connected to the array elements by transmission lines which also propagate TEM waves. As a result, the phase lengths of paths from the input ports to the antenna elements vary in proportion to frequency, giving a beam direction independent of frequency. The invention described in U.S. Pat. No. 3,868,695 will also produce beams with directions independent of frequency if it is implemented with power dividers and delay lines having TEM wave characteristics.
Other examples of prior art systems are illustrated in, for example, U.S. Pat. No. 2,817,084, Clapp et al, Dec. 17, 1957, U.S. Pat. No. 3,085,204, Sletten, Apr. 9, 1963, U.S. Pat. No. 3,271,776, Hannan, Sept. 6, 1966, U.S. Pat. No. 3,308,468, Hannan, Mar. 7, 1967, U.S. Pat. No. 3,731,316, Sheleg, May 11, 1973, U.S. Pat. No. 3,736,592, Coleman, May 29, 1973, and U.S. Pat. No. 3,877,014, Mailloux, Apr. 8, 1975.
The '084 patent teaches a junction for feeding antenna elements 31, 32 and 33 through lines 21, 22 and 23 respectively from a main transmission line 24. However, the '084 patent teaches a matching section 25 at the junction of the branch transmission lines 21, 22 and 23 and the main transmission line 24.
In the '204 patent, and especially in FIG. 4, a source is connected to nine antenna elements through various paths which appear to be coupled at simple junctions. However, only a single source is feeding all of the antenna element arrays.
The '776 patent shows an arrangement wherein all of the branch transmission lines 15, 16, 17 and 18 are intercoupled by intercoupling lines 22 to 26. This is for the purpose of impedance matching of array antennas.
The '468 patent, by the same inventor as the '776 patent, shows a plurality of outputs being fed to each one of the elements of an antenna array. However, they are fed to the elements through various hybrid junction devices such as the devices 49 and 50 in FIG. 6.
The '316 and '592 patents include teachings relative to Butler Matrices. The '014 patent includes teaching of a single beam forming circuit 6 which has an output connected to each element of an antenna array.